1. Field of the Invention
The present invention relates generally to solid state lasers, and more particularly relates to the polarization switching of vertical cavity surface emitting lasers, or VCSELs.
2. Description of the Related Art
Optical interconnections and transceivers are currently being used to provide reliable interconnections between electronic components that can scale in both distance and speed. The VCSEL technology has had a substantial impact on this industry as a low-cost, wafer-scale, and high-speed device that can be directly driven by low-cost silicon circuits. For reasons of manufacturing cost, packaging costs and performance, current-injection (or current-modulation) VCSELs have dominated the low-cost, short reach markets for transceivers in the 1-10 Gigabit per second (Gbit/s) range. However, these conventional current-injection VCSELs are, for numerous reasons, bandwidth limited.
For example, the current-injection VCSELs exhibit RC limits due to charging and discharging of the VCSEL and electrode (i.e., the contacts) capacitances. Electrode capacitance can be eliminated by suitable design of non-overlapping intra-cavity contacts placed on opposite sides of the cavity to avoid lateral electrode overlap. However, VCSEL intrinsic capacitance (i.e., the active-layer capacitance), to date, cannot be easily reduced. While the intrinsic bandwidth (fmax) of VCSELs, based on their fundamental material properties, can theoretically be in excess of 90 Gigahertz (GHz), the RC limits due to the charging and discharging of the VCSEL and electrode capacitances have limited the operating bandwidth to about 20 GHz for even the fastest conventional VCSELs. Additionally, the conventional current-injection VCSEL experiences detrimental carrier transport effects related to movement and re-distribution of carriers in active region. Further, heating effects caused by current modulation reduces the intrinsic bandwidth (fmax) of a VCSEL. Finally, mode-competition negatively effects the multi-modal VCSELs.
Conventional VCSEL structures also typically have random polarization states. Much work has been done in the art to attempt to understand the dynamics of the polarization behavior of VCSELs. In a typical prior art VCSEL, as the injected current is varied, the polarization state can exhibit hysteresis and noisy behavior. This makes the polarization state of the output light difficult to predict and control. However, it is known that one may “fix” the polarization state of a conventional VCSEL by introducing an asymmetry into the cavity structure through the use of mechanical strain. Mechanisms used to apply the strain in different directions in order to switch the polarization have been proposed. However, due to the need to mechanically alter the stress, these proposed switching mechanisms are inherently slow.
It is also known in the art that by designing a VCSEL with a rectangular aperture, the polarization state of the output light prefers to align along the direction of the longer axis of the rectangular aperture. Further, polarization switching VCSEL designs based on the intersection of such rectangular aperture regions and switching current flow along the corresponding longer axes have been proposed. Aside from even more complex aperture geometries and associated processing complexities, such designs necessitate a more substantial movement and re-distribution of carriers in the active region of the VCSEL, which tends to reduce the maximum rate of switching between the preferred polarization states.
It has also been observed that it is possible to substantially fix the polarization of a square cavity VCSEL by using asymmetric non-overlapping electrodes to preferentially inject current along one lateral axis of the cavity. The proposed structure used a multi-layer Distributed Bragg Reflector (DBR), and one generalized embodiment is shown in FIG. 1. As shown in FIG. 1, both top and bottom DBR mirrors are used, which makes the fabrication of such a VCSEL with one pair of dual intra-cavity contacts difficult and the fabrication of such a VCSEL with two pair of dual intra-cavity contacts extremely difficult.
FIG. 2 illustrates another generalized example of a prior art VCSEL structure. As shown in FIG. 2, the VCSEL structure includes one pair of intra-cavity contacts (i.e. one p-contact 202 and one n-contact 204). A top mirror 206 may be a deposited dielectric for maximum processing flexibility and VCSEL reliability. This contact is in stark contrast to the top DBR mirror used in the device shown in FIG. 1. The bottom mirror 208 of the FIG. 2 device can be epitaxially grown and is a semiconductor DBR. The n-contact 204 is disposed on the bottom mirror 208. The VCSEL mesa includes the active quantum-well region 210, a current confinement aperture 212, and the remainder of the optical cavity 214. The p-contact 202 is disposed on top of the VCSEL mesa. The dielectric top mirror 206 is placed as the last step of the fabrication process, completing the cavity structure. The pair of contacts 202, 204 are intra-cavity because they bypass the mirror pairs.
Therefore, what is needed is a VCSEL that provides fast switching of the polarization state without the limitations of the conventional art.